Microporous membranes using polyolefins as raw materials are widely used as separation membranes, permselective separation membranes, separation materials, and the like for various substances. Examples of their applications include microfiltration membranes, fuel cell separators, capacitor separators, the base materials of functional membranes in which pores are filled with a functional material to exhibit a new function, and battery separators. Among them, as separators for lithium ion batteries widely provided in notebook personal computers, cellular phones, and digital cameras and for in-vehicle use and the like, polyolefin microporous membranes are preferably used.
In recent years, the performance enhancement of these electronic apparatuses has also been significant, and with this, particularly the demand for higher capacity of batteries has increased rapidly. Under the circumstances, excellent permeability and further improvement of the fuse effect are required of lithium ion battery separators. The fuse effect is a mechanism in which when overheating occurs inside a battery due to an overcharged state or the like, the separator melts and forms a film covering the electrode to interrupt the current, thereby ensuring the safety of the battery. In the case of a polyethylene microporous membrane, the fuse temperature, that is, the temperature at which the fuse effect is exhibited, is known to be roughly around 140° C., but from the viewpoint of stopping a runaway reaction or the like inside a battery as quickly as possible, lower fuse temperature is considered to be better.
Therefore, Patent Literature 1 proposes a polyethylene microporous membrane excellent in mechanical strength and permeability and having low fuse temperature and high heat resistance.